The first volume in J.K. Rowling's blockbuster series about young wizards living in a magical modern world is a perfect example of the difficulties in finding exact numbers for a list of this nature. It was first published in 1997, fully subject to modern sales tracking methods. However, with different publishers for different nations, and various publishers reporting seemingly exaggerated numbers, it's more accurate to say, "107 million, give or take 20 million or so."